Conductive liquid developer for xerographic images



1957 K. M. OLIPHANT 3,

' I CONDUCTI VE LIQUID DEVELOPER FOR XEROGRAPHIC IMAGES Filed Feb. 25, 1962 INVENTOR. Kern-4 MERmrrH O\ \PHANT BY. W %M 3,300,416 CONDUCTIVE LllQUll) DEVELOPER FOR XEROGRAPHIC IMAGES Keith Meridith Oliphant, Westbourue Park, South Australia, Australia, assignor to Research Laboratories of Australia Limited, Adelaide, South Australia, Australia Filed Feb. 23, 1962, Ser. No. 174,989 Claims priority, application Australia, Mar. 13, 1961,

,370/61 3 Claims. (Cl. 252-621) This invention relates to liquid developers for electrostatic images with particular reference to the development of electrostatic images on plates or papers and other backings of the kind used in electrophotographic processes wherein an image is produced on an electrostatically charged plate by purely electrical or by photographic means. The invention also relates to those processes where a sensitive layer is charged electrostatically and then exposed to form a latent image which is developed by a developer to give a powder image which may then be transferred to a sheet of transfer material. The invention has for its purpose to afford a developer composed of material which is conductive when deposited and acts as a shield to protect the underlying sheet against subsequent charging so that subsequent charging is constrained to occur in the unshielded areas.

Developers previously used for the purpose comprised metallic particles and the like deposited on the photoconductor surfaces in pattern form for the purpose of shielding the surface beneath the deposited particles ied States Patent against the effect of charging which would ordinarily be achieved for example by submitting the photoconductive sheet to a corona discharge from an array of needles or wires. This shielding is effected by using a developer containing conductive particles which when deposited from the developer carrier liquid or fluid will act as the necessary conductive shielding medium. At the present time it is customary to use particulate material or powder which is milled or otherwise'coated with a resin and to suspend this in a liquid of high electrical volume resistivity and low dielectric constant whereby the particles are free to deposit under the influence of and in the direction of the electric field associated with the image or electrostatic pattern.

One use for developers of this type is in the production of reversed images in Xerography or electrophotography where it is necessary to first develop the xerographic image by depositing the developer which will thereafter act to mask the areas on which it is deposited and then to recharge the areas on which the developer particles are not present or present to a lesser degree and to redevelop the so-formed image with a colour which will produce the required appearance of reversal, said second developer having a darker hue than said conductive developer. At the present time it is customary to make a deposition first by using a developer having a colour such as white and then to eifect a second development on the opposite areas with a darker colour or simply to use a black developer in the second development the image in this way being reversed in that the electrostatic image which is first developed is produced in a light colour while the second image is thereafter developed in a darker colour in those areas not fully masked by the first image deposit. Sometimes it is taught to remove the first developer deposit during the process.

One disadvantage of prior art reversal processes of this kind is that the image in the first development comprises particulate material which limits resolution because of limitations in particle size because large particles are necessary otherwise the development results only in partial shielding where full shielding is desired. Another dis- "ice advantage is that the first development is carried out with a developer which is coloured or at least solid material of white colour and all of this material may require removal or removal in part in order to reveal the finished reversal picture. A further disadvantage is the limitation that the developer particles must themselves comprise material which is intrinsically conductive of itself and this excludes material which is intrinsically insulating.

Accordingly one object of this invention is to provide an improved developer containing conductive material which comprise colourless or transparent substances available to deposit from suspension to form an image of an invisible kind which subsequently controls the deposition of a second developer by electrically shielding the underlying surface. Another object of the present invention is to provide a conductive developer having improved resolution. A still further object of thi invention is to provide a conductive developer for reversal of images which is characterised in that it is not necessary to remove the developer material of the first step when the surface has been subjected to development in the second step.

The objects of the invention are achieved by utilizing a resin or similar film-forming material for the first development, said film forming material having been rendered at least temporarily conductive by the addition thereto of a material with a low volume resistivity, such as a polar solvent, in such a manner that sufiicient solvent is retained in or on the resin or the like to render the film produced therefrom relatively conductive when deposited.

In general the invention is carried into effect by dissolving a suitable resin in a strong polar solvent or by subjecting the resin to vapour of said solvent or by permitting said resin to otherwise imbibe said solvent by contact between the resin and the solvent such as for example the solvent methyl iso-butyl ketone after which the solution so formed is subjected to the action of a weaker non-polar solvent which is soluble in the methyl iso-butyl ketone but has a lower solubility for the resin so as to cause precipitation of the resin from the mixture because of the lowered solubility of the resin in the mixture.

Aprecipitation of the resin then occurs, said precipitate or gel retaining a certain proportion of the original polar solvent absorbed or adsorbed by molecular groups of the resin and this retained polar material renders the resin precipitate conductive or semi-conductive. The resin precipitate so produced is then dispersed in a liquid vehicle characterised by high electrical resistivity and low dielectric constant and the dispersion is used thereafter as a developer for electrostatic images the dispersion being such that the resin is available in a mobile colloidal form in the carrier liquid so as to allow the resin to be deposited electrically from the body of the liquid according to the electrostatic field relating to the latent electrostatic image existing on the photoconductive or other surface.

When the invention is applied to the production of reversed xerographic images a great adwance in the art results from the process in that the first development is carried out with a pigment-free film so that if a white photoconductor surface is used the first development will exist as a resin which willnot affect the colour of the photoconductor surface in that no pigment material is added and thus it is that when the second development takes place with a normal pigmented developer the image will be similar in appearance to the image which would be obtained by direct development. p

In further explanation of this invention the resin is dissolved in a polar solvent or in a solvent with relatively low volume resistivity and this solution. is reduced in solvent power thereafter by the addition of a poor solvent and this results in precipitation of the resin. This process of precipitation may be repeated a number of times or other polar solvents may be used for causing precipitation so as to eliminate any excess of polar solvent or low volume resistivity solvent in the final vehicle because the presence of polar solvent in the final carrier liquid requires control so that it does not lower the volume resistivity of the total liquid beyond the limit allowable for liquid developers. It is known for instance that liquid developers must utilize a carrier liquid having a volume resistivity in excess of ohm-cm. and a dielectric constant less than three and the polar solvent therefore must be sufificiently absorbed or adsorbed into the resin to avoid the presence of free polar solvent in the carrier liquid to an extent which would render the carrier liquid conductive and capable of bleeding away the electrostatic latent image during development. It is desirable therefore to use a resin having poor solvent release properties and for this reason examples of the process which are referred to herein include resins which do not release the absorbed or adsorbed solvent readily.

It will be seen from the examples which are referred to herein that the improved process has particular refer ence to the use of resins which are themselves colourless or substantially colourless and which would not in themselves be electrically conductive to the extent of masking an area of photoconductor surface against recharging but when such resins are treated with a conductive polar solvent or imbibe said solvent they will provide a conductive medium which will blanket the photoconductor beneath the resin deposit and will prevent said photoconductor from taking a surface charge when subject to a corona discharge or the like when applied in the customary way for charging insulator surfaces. As no pigment material is contained in the developer but only the resin which contains entrapped solvent of conductive type it will be realized that a colourless first developer can be put down on the photoconductive surface which will have the necessary characteristic of providing a first developer for a xerographic reversal process or printed circuit but it is clear however that the resin can at some later time lose its conductivity when the entrapped or imbibed polar solvent evaporates after development of the image and completion of the reversal of the image.

The invention thus allows a photoconductor surface such as for example a zinc oxide dispersed in a resin to retain a normal colouration and when a reversed image is produced by the present method only one development will be apparent namely the second development this being however deposited in the opposite or reverse areas to the original optical or X-ray or electrostatic image by means of which the first conductive developer material was attracted to the photoconductive surface.

For the better understanding of the invention reference may be made to the accompanying figures in which one embodiment of the invention is set out.

In these drawings, FIGURE 1 shows a schematic dia gram of a sheet covered with an insulator or photoinsulator or photoconductive coating.

FIGURE 2 shows this sheet after first development with a conductive developer present in the image areas.

FIGURE 3 shows this sheet after recharging by means of a corona discharge with charges deposited in the nonimage areas, and

FIGURE 4 shows the sheet after the second development in which a known xerographic type of developer is deposited in the charged areas of FIGURE 3, to produce a reversal of the original image.

In FIGURE 1 a sheet or backing of metal or wood or paper or other material is coated with an insulator material such as a film of resin or a photoinsulator such as selenium applied for example by vacuum evaporation or a layer of a photoconductor such as zinc oxide embedded in an insulating film such as for example an alkyd resin or an epoxy resin. The sheet is designated 1 and the coating thereon is designated 2.

FIGURE 2 shows said sheet 1 coated with said photoconductive coating 2 which has been subjected to an over all electrostatic charge which has been bled away by exposure to light or may consist merely of an electrostatic image produced by differential charging, so as to produce in the areas 5 and 6 charged Zones which will attract the conductive developer material 3 and 4 in which 3 represents the resin and 4 the adsorbed or imbibed polar solvent which is pictured as a continuous film on the top of the resin layer but may actually consist of particles or globules of resin having a coating or outer layer of conductive or polar solvent thereon.

FIGURE 3 shows the sheet 1 and coating 2, surmounted in the image areas 5 and 6 by the developer material 3 and 4 said sheet 1 has then been recharged overall by a corona discharge device but has retained charge only in the non-image areas designated 7 and 8.

FIGURE 4 shows the sheet 1 and coating 2 carrying the first developed image 3, 4 and the second developed image 9 and It) in the areas which were recharged and developed thereafter designated 7 and S, the second developed image 9 and 19 comprising a reversal of the original optical image or electrostatic image. The final sheet is composed thereof of a first image 3, 4 which is conductive and transparent and colourless and a second reversed image 9, 10 which may be composed of any ordinary xerographic developer material. The polar solvent 4 may be allowed to evaporate if desired following which the resin first image may be retained as an insulating film which is invisible to the eye.

The following examples show the invention may be applied in practice but are not to be taken as limitations of the invention as any polar solvent or any solvent with relatively low volume resistivity can be used in association with any resin capable of dissolving and holding the polar solvent at least during the first and second developments so that the presence of the polar solvent renders the resin at least temporarily conductive or semi-conductive. It will be obvious also that a dye or colouring medium may be incorporated with the resin or the polar solvent if it is desired to produce for any reason a coloured first development.

Example 1 In the first example of the invention the resin which was used was epichlorohydrin resin supplied under the trademark Epikote 850 by Shell Co. This resin is supplied in solution in solvents which include butyl Cellosolve (ethylene glycol monobutyl ether) which is a polar solvent. The first step in formulation was to produce a solution of epichiorohydrin resin in the polar solvent by evaporating to 60 percent solids. In the second step this solution was dispersed in a hydrocarbon solvent having a Kauri Butanol value of 40, an aniline point of 49, containing 8 percent aromatics as known under the trademark Shell Solvent X55 by vigorously shaking the resin in the solvent at room temperatures, e.g., 20 C. in the proportions 1 part by volume of the resin solution with 50 parts of the solvent with the result that a fine shock suspension of the resin was set up in the X55 solvent in which the resin particles have adsorbed on them a layer of butyl Cellos-olve. This suspension is used as as a developer for an electrostatic image giving a resolution of more than 50 lines/mm. For reversal processing the dried image-bearing surface is recharged and blanket developed in which case the charge is not retained on the conductive resin areas and a reversal results.

Example 2 A conductive resin dispersion was prepared from a short oil type linseed oil modified alkyd resin known under the tradename, Rhodene L9/50 supplied by Polymer Corporation Ltd., in which the alkyd resin was dissolved as a 50 percent solids solution in non-volatile aromatic hydrocarbon solvents. Rhodene L9/50 has an oil length of 40%, an acid value of 25-35 and specific gravity at 20 C. of 0.98-0.99. This solution was diluted with 2 parts by volume of acetone to 1 part of solution and was then shock precipitated at 20 C. or in the range C. to 50 C. by the addition of 10 parts by volume of an aliphatic hydrocarbon such as the solvent known under the trademark Shell Solvent X55. The excess liquid was then decanted and the precipitate re-dissolved by the addition of 10 parts of toluene to 1 part of precipitate by volume. To obtain the desired suspension of conductive resin 5 cc. of this solution was added to 150 cc. of the above aliphatic hydrocarbon solvent at 20 C. whereupon precipitation of the resin as fine conductive particles occurred. This suspension was then used to develop a colourless conductive image on a photoconductive sheet so as to control a second development with an ordinary liquid developer so as to form a reversed image, said first developement giving a resolution of more than 50 lines per mm.

Example 3 In the first step of this example an epoxyester resin was used. The particular resin was one marketed under the trade name Beckosol P786, by Reichhold Chemical, which is an epoxyester resin containing 42 percent dehydrated castor oil having an acid value of 3 maximum in solution in xylol to the extent of 50 percent solids. This was diluted with 4 parts of acetone in which the resin is soluble and then shock precipitated by the addition at 20 C. of 10 parts of an aliphatic hydrocarbon liquid such as that known under the trademark Shell Solvent X4. The excess liquid was decanted and the precipitate re-dissolved with toluene in the proportions 10 parts by volume of toluene to 1 part of precipitate. A conductive resin dispersion was obtained by adding the resultant solution to the above alpihatic hydrocarbon liquid in the proportion 1 part of solution to 30 parts of the liquid by volume. The resultant suspension was used as a colourless first developer in the process of reversal of an electrostatic image giving a resolution of more than 50 lines/ mm. for the first developer.

It will be seen from these examples that the invention resides in the production of a liquid developer for electrostatic images characterized in that a resin is first dissolved in or has dissolved in itself a polar solvent or a solvent which dissociates and therefore is able to conduct or transport electricity or to cause the collapse of an electric field relating to an image charge, said solution of the resin in the polar solvent or of the polar solvent in the resin as the case may be being thereafter dispersed having regard to temperature in a non-polar or insulating liquid of the type which it is well known now to use in the art of liquid development so as to produce a liquid developer for electrostatic images wherein the particles available for development are resinous fine particles or colourless colloid film-forming particles characterised by having a high electrical conductivity or high dielectric constant or dissociating character which will not support a charge after deposition of these particles on the surface of the photoconductive sheet but characterized by being inert for the purposes of liquid development in that the property of conductivity of the surface of the particles does not impair the charge supporting properties of the carrier liquid in which they are dispersed and thus are able to migrate under the influence of the electric field associated with the image charge due to dielectric effects or to electrophoresis so as to produce a colourless reproduction of the image on the surface of the sheet which will then serve to control subsequent charging steps in that the developed areas will not accept a charge whereas the undeveloped areas will with the result that a reversal of the original optical or electrical image occurs, said reversal image having an improved appearance and showing no evidence of the manner in which it was achieved.

What I claim is:

1. For developing electrostatic images, a developer consisting essentially of an electrically insulating carrier liquid having a volume resistivity in excess of 10 ohm cm. and a dielectric constant less than 3, film-forming resin particles suspended in said carrier liquid in the volumetric proportion of from about 0.1 to 1.5 parts of said resin to parts of said carrier liquid, and an electrically conductive at least partial solvent for said resin and having a lower volume resistivity than said film-forming resin, wherein the volume resistivity of said partial solvent is below 10 ohm cm. and its dielectric constant is in excess of 3, said partial solvent being adsorbed onto the resin particles and being present in a quantity just sufficient to be fully adsorbed by said suspended resin particles but not sufiicient to dissolve or disperse in said carrier liquid so as to lower substantially the volume resistivity thereof.

2. A developer according to claim 1 wherein the resin is selected from the group consisting of linseed oil modified alkyd resin and epoxy ester resin containing dehydrated castor oil.

3. A developer according to claim 1 wherein the resin particles are colloidal in size.

References Cited by the Examiner UNITED STATES PATENTS 2,907,674 10/1959 Metcalfe et a1. 25262.1 2,939,804 6/1960 Schossberger et a1. 25262.1 2,945,825 7/1960 Coler 252--511 3,005,726 10/1961 Olson 11737 X 3,135,695 6/1964 York 252-62.1

OTHER REFERENCES Weiser, Colloid Chemistry, second edition (1949), page 146.

LEON D. ROSDOL, Primary Examiner.

JULIUS GREENWALD, ALBERT T. MEYERS,

Examiners.

S. R. BRESCH, J. D. WELSH, Assistant Examiner. 

1. FOR DEVELOPING ELECTROSTATIC IMAGES, A DEVELOPER CONSISTING ESSENTIALLY OF AN ELECTRICALLY INSULATING CARRIER LIQUID HAVING A VOLUME RESISTIVITY IN EXCESS OF 10**9 OHM CM. AND A DIELECTRIC CONSTANT LESS THAN 3, FILM-FORMING RESIN PARTICLES SUSPENDED IN SAID CARRIER LIQUID IN THE VOLUMETRIC PROPORTION OF FROM ABOUT 0.1 TO 1.5 PARTS OF SAID RESIN TO 100 PARTS OF SAID CARRIER LIQUID, AND AN ELECTRICALLY CONDUCTIVE AT LEAST PARTIAL SOLVENT FOR SAID RESIN AND HAVING A LOWER VOLUME RESISTIVITY THAN SAID FILM-FORMING RESIN, WHEREIN THE VOLUME RESISTIVITY OF SAID PARTIAL SOLVENT IS BELOW 10**9 OHM CM. AND ITS DIELECTRIC CONSTANT IS IN EXCESS OF 3, SAID PARTIAL SOLVENT BEING ADSORBED ONTO THE RESIN PARTICLES AND BEING PRESENT IN A QUANITY JUST SUFFICIENT TO BE FULLY ADSORBED BY SAID SUSPENDED RESIN PARTICLES BUT NOT SUFFICIENT TO DISSOLVE OR DISPERSE IN SAID CARRIER LIQUID SO AS TO LOWER SUBSTANTIALLY THE VOLUME RESISTIVITY THEREOF. 